Components of high temperature turbine engines are often manufactured from nickel-, cobalt-, or iron-based superalloy materials, which are recognized as providing greater shape retention and strength retention over a wider range of operating temperatures than other candidate materials for these applications. Although superalloy materials exhibit improved mechanical properties at high operating temperatures, they are nonetheless susceptible to high temperature oxidation and hot corrosion. While the efficiency of a turbine engine generally increases with increasing operating temperature, the ability of superalloy materials to operate at such increased temperatures is limited by the ability to withstand such oxidation and corrosion.
Generally, gas turbine engines include a compressor for compressing incoming air, a combustor for mixing the compressed air with fuel, such as jet fuel or natural gas, and igniting the mixture, and a turbine blade assembly for producing power. In particular, gas turbine engines operate by drawing air into the front of the engine. The air is then compressed, mixed with fuel, and combusted. Hot combustion gases from the combusted mixture pass through a turbine, which causes the turbine to spin and thereby power the compressor.
External surfaces of superalloy turbine engine components, which may experience direct contact with the hot combustion gases, are susceptible to high temperature oxidation and hot corrosion that accelerates the oxidation process. These external surfaces are frequently provided with an intermetallic or aluminide overlayer or diffusion coating that protects the underlying superalloy material against high temperature oxidation and hot corrosion by forming a stable thermal oxide scale. High temperature oxidation and hot corrosion, if the temperature is sufficiently high, may form corrosive deposits which attack and degrade the protective scales.
At lower operating temperatures, external surfaces of superalloy turbine engine components are susceptible to a form of hot corrosion known as sulfidation corrosion. Sulfidation corrosion is most frequently observed on turbine engine components operating below 1500° F. Sulfidation corrosion forms deposits on the external surfaces of the superalloy material in the form of a metal sulfide scale.
Internal surfaces of a turbine engine component may not be protected by either an aluminide or chromide overlayer or a diffusion coating. In addition, these internal surfaces are subject to a significantly different service environment than the external surfaces of the turbine engine component. For example, internal passages in the turbine engine component may be supplied with a flow of bleed air from the engine compressor, rather than combustion gas, for reducing the operating temperature. Consequently, when the gas turbine engine is operating, the internal surfaces bounding these passages are at a lower temperature, typically about 1100° F. or cooler, than the external surfaces. As a consequence, sulfidation corrosion may cause deposits to form on the surfaces of these internal passages. The formation of sulfide scales incorporates nickel-based superalloy material from the walls, which operates to thin the wall sections of these internal passages. Progressive thinning may eventually lead to component failure from overly thin material in the wall sections.
Another type of deposit observed in the internal passages of turbine engine components is dust particles consisting, for example, of a mixture of silica, alumina, and calcium sulfate. During service in a turbine engine, these indigenous dust particles, which are commonly known as runway sweepings or Arizona Road dust, may infiltrate into internal passages of turbine engine components and deposit on the interior surfaces. Silicon and sulfur from the dust particles may diffuse into the turbine engine component, which may reduce the melting point of the superalloy material or may precipitate the occurrence of sulfidation corrosion. Because deposits that form or accumulate on the internal surfaces of turbine engine components, whether originating from sulfidation corrosion or dust particles, cause damage, it is desirable to remove these deposits with a cleaning process.
Batch thermo-chemical cleaning processes may be used to clean the external surfaces of turbine engine components. One such process known as fluoride ion cleaning (FIC) generally relies on the high reactivity of fluorine or fluoride ions for cleaning the exterior surface. However, cleaning internal surfaces of the turbine engine component differs dramatically from cleaning the external surfaces. Because of the flow restrictions imposed by the small diameter of the internal passages (typically from about 0.1 inch to about 0.5 inch), FIC cleaning may be ineffective for removing deposits from the interior passages. FIC cleaning would require piping to conduct the gases from a FIC cleaning reactor to the internal passages, which raises the cost of the cleaning system. In addition, FIC cleaning is a relatively hazardous and environmentally unfriendly method of cleaning turbine engine components. FIC cleaning is not customarily used on turbine blades because of the tendency to cause intergranular attack on the blades, which could lead to cracking and to eventual catastrophic failure.